Technique for electroetching thin film metallization

ABSTRACT

PATTERN DELINEATION OF THIN FILM METALLIZATIONS INCLUDING GOLD, PLATINUM AND RHODIUM IS EFFECTED ELECTROLYTICALLY IN THE PRESENCE OF A STONG ACID HAVING A PH LESS THAN ONE. THE DESCRIBED TECHNIQUE PERMITS CONTROLLED ETCHING OF NOBLE METALS WHICH HERETOFORE HAVE BEEN ETCHED ONLY WITH DIFFICULTY.   D R A W I N G

J. YAHALOM -March 19, 1974 TECHNIQUE FOR ELECTROETCHING THIN FILMMETALLIZATION Filed Oct. 19, 1972 FIG.

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ES V E2 @555 0 W 0 E w W T F. B) 0 E 0 6H 1 N( M Lm U 0 P l 4 C2 H+ 00 T0 w H 0 nu m Q W 0 0 0 O 0. Q 0 Q 0 0 Q 0 0 O 4 3 2 PULSE DURATION (SEC)3,798,141 Patented Mar. 19, 1974 United States Patent Office 3,798,141TECHNIQUE FOR ELECTROETCHING THIN FILM METALLIZATION Joseph Yahalom,Maplewood, N.J., assignor to Bell Telephone Laboratories, Incorporated,Murray Hill, NJ. Filed Oct. 19, 1972, Ser. No. 298,856 Int. Cl. C23b3/02 US. Cl. 204-12943 7 Claims ABSTRACT OF THE DISCLOSURE Patterndelineation of thin film metallizations including gold, platinum andrhodium is effected electrolytically in the presence of a strong acidhaving a pH less than one. The described technique permits controlledetching of noble metals which heretofore have been etched only withdifiiculty.

This invention relates to a technique for electrochemically etching thinfilm metallizations. More particularly, the present invention relates toa technique for selectively etching thin film metallization includingnoble metals by electrochemical means.

In recent years miniaturization of components and circuitry coupled withthe increasing complexity of modern electronic systems have created anunprecedented demand for reliability in thin film circuitry and the needfor the total exploitation of the technology. This is particularly truein the case of lead attachment which has long been recognized as being acritical factor in the stability of circuit characteristics.

Early workers in the art recognized that the metallurgical compatibilityof the various metallic constituents of the joining and conductingsystem played a prominent role in determining the parameters ofinterest, so motivating the use of a single metal for this purpose.Although such systems were found to be ideal from a metallurgicalstandpoint, they sulfered from inherent defects in that the manufacturerwas necessarily restricted from the standpoint of obtaining optimumcircuit characteristics. Accordingly, the interest of workers in the artwas focused upon multimetal joining or conducting systems.

Unfortunately, studies have revealed that thermal degradation ofadhesion of gold-transition metal thin film composites often occurs,such being attributed to diffusion and migration affects. In order toobviate this limitation, workers in the art proposed inserting abarrier, typically platinum or palladium between the gold and the othermember of the composite. Although rhodium was also considered for thispurpose, attempts at pattern delineation proved fruitless. 7

Studies of composites prepared utilizing palladium as the barrier layerrevealed that interdiflusion between the gold and the palladium occurredat high temperatures, thereby resulting in a reduction in theconductivity of the gold and altering circuit parameters.

Although platinum has not proven as convenient to use as palladium, ithas not evidenced the interdiffusion problem with gold. However,platinum was found to be very diflicult to etch and the only materialfound suitable for this purpose has been aqua regia, a material whichattacks photoresists. Further investigation revealed that patterndelineation of the platinum containing composite could only be effectedby back sputtering, a technique which is economically undesirable andwhich often results in the deposition of residues. Still further, backsputtering could not be used in the fabrication of MOS devices due tothe likelihood of damaging the dielectric. Accordingly, the platinumcomposites have been utilized primarily in the fabrication of bipolardevices. Recognizing these limitations, workers in the art have focusedtheir interest upon the development of a technique suitable for patterndelineating all noble metals with a universal etchant.

In accordance with the present invention, this end has been attained bymeans of a novel electrochemical etching technique Briefly, theinvention involves etching at least one thin film material selected fromamong gold, rhodium and platinum in a non-oxidizing electrolyte having apH less than one by applying a potential difference between the sampleto be etched and an auxiliary electrode so as to develop a potential onthe sample with respect to a reference electrode which variesperiodically between anupper and a lower value. In a preferred embodiment, etching of a gold-transition metal metallization compositehaving a barrier layer selected from the group consisting of platinumand rhodium is effected selectively in an electrolyte comprising anon-oxidizing acid having a pH less: than one. Composites may be etchedin the described nianner in controllable fashion at a significantlyhigher rate than that attainable by conventional electroetchingtechniques, the metals being removed selectively by adjusting the heightof the potential.

The invention will be more readily understood by reference to thefollowing detailed description taken in conjunction With theaccompanying drawing wherein:

FIG. lis a schematic representation of a typical system suitable for usein the practice of the present invention; and

FIG. 2 is a graphical representation on coordinates of pulse duration inseconds against etching rate in Angstroms per minute for atitanium-platinum-gold composite pulse etched in accordance with thepresent invention.

With reference now more particularly to FIG. 1 there is shown an etchingvessel 11 having disposed therein thin filmj composite 12 comprising aninsulating substrate 13 having deposited thereon a transition metal orglue layer 14; a noble metal 15 selected from among platinum andrhodium, and patterned gold layer 16. An electrolyte 17 is employed inan amount sufiicient to cover the metallized composite. Electrolyte 17is a non-oxidizing acid having a pH less than one. Typical acidssuitable for use in the practice of the present invention arehydrochloric, sulphuric, hydrofluoric, phosphoric and the like. Theelectrical system employed includes contacting electrode 18 which isconnected to layer 14, electrode 18 being insulated from electrolyte 17by means of insulating sheath 19. A second electrode 20 functions as thecounter or auxiliary electrode in the system and is immersed inelectrolyte 17. The third electrode 21 is a conventional referenceelectrode, typically a calomel electrode, which serves to maintain thecomposite 12 at a controlled potential with respect to the electrolyte,thereby permitting control of the etching process. Each of theelectrodes is shown connected to a pulsing means 22, typically apotentiostat including a pulse generator.

In the operation of the process, the apparatus depicted in FIG.-;l isset up and etching initiated by applyingv a difference of potentialbetween the sample and the auxiliary electrode so as to develop apotential on the sample with respect 'to the reference electrode whichvaries periodically between an upper and lower value. With respect tothe composite described herein, this potential may range from 1450 to1600 mv. (hydrogen scale) at the upper value and from 550 to 60 mv.(hydrogen scale) at the lower value. Studies have revealed that for slowetching, the lower value may range from 55.0 to 240 mv. and from 240 to60 mv. for rapid etching. It has also been found that the describedtechnique may be used selectively with respect to platinum, gold andrhodium. Thus, the use of higher potentials ranging from 1450 to 1600mv. results in the etching of gold, platinum and rhodium. However,

rhodium may be etched selectively with respect to platinum and gold bymaintaining the upper value of potential between 900 and 1100 mv. andrhodium and platinum selectively etched with respect to gold at upperpotential values ranging from 1100 to 1450 mv. (all potential being onthe hydrogen scale). The upper limit of 1600 mv. is dictated byconsiderations relating to undesirable gas evolution. The lowerpotential limit is determined by etching rate considerations. The periodof time 'that the sample is maintained at either the upper or lowerpotential may conveniently range from 1 to milliseconds, the shortertime periods corresponding with the higher etching rates and theconverse. It has been found that platinum may he etched at 1 milliseconddurations at the rate of 4000 A. per minute and rhodium at 2000 A. perminute. However at 10 millisecond durations etching of platinum may beeffected at the rate of 500 A. per minute and rhodium at the rate of 250A. per minute.

With reference now to FIG. 2, there is shown a graphical representationon coordinates of duration of time that the sample is maintained ateither the upper or lower potential with respect to the referenceelectrode versus etching rate in Angstroms per minute for atitaniumplatinum-gold thin film composite etched in a 25 percenthydrochloric acid solution to potentials (on the hydrogen scale) from-60 to 240 mv. It will be noted that a one millisecond duration resultsin etching of platinum at 4000 A. per minute and at 10 milliseconds at500 A. per minute. It will be understood that removal of the base orglue layer may also be effected by lowering the potential to a constantnegative value, or by etching to a potential between 500 and 300 mv. atdurations ranging from 1 to 10 milliseconds, so resulting in a universaletching bath. A final chemical etching step will, of course, be used asrequired to remove detached residues.

Examples of the present invention are described in detail below. Theseexamples are included merely to aid in the understanuing of theinvention and variations may be made by one skilled in the art withoutdeparting from the spirit and scope of the invention.

EXAMPLE 1 A composite comprising successively 500 A. titanium, 500 A.platinum and 10,000 A. of gold deposited upon a ceramic substrate memberwas immersed in an apparatus of the type shown in FIG. 1, a 25 weightpercent hydrochloric acid solution having a pH of about 1 being employedas the electrolyte. The auxiliary electrode was comprised of carbon andthe reference electrode was a calomel electrode. Etching was theneffected by establishing a potential of -60 mv. on the sample withrespect to the reference electrode for a duration of 1 millisecond andthen establishing a potential of 1600 mv. for a duration of 1millisecond. The procedure was continued successively until the gold wasremoved. Then, the procedure was repeated using a lower potential valueof 60 mv. and an upper value of 1240 mv. to remove the platinum at arate of 4000 A. per minute.

EXAMPLE 2 The procedure of Example 1 was repeated using a duration of 10milliseconds. The platinum was etched controllably at the rate of 500 A.per minute.

EXAMPLE 3 EXAMPLE 4 The procedure of Example 2 was repeated using acomposite comprising 500 A. of rhodium rather than platinum, the rhodiumbeing etched using a duration of 10 milliseconds at a rate of 250 A. perminute.

What is claimed is:

1. Technique for electrochemical etching of a thin film selected fromthe group consisting of gold, platinum and rhodium which comprisesimmersing said thin film together with an auxiliary electrode and areference electrode in an electrolyte comprising an acid having a pHless than one and etching the thin film by applying a difference ofpotential between the thin film and the auxiliary electrode so as todevlop a potential on the thin film with respect to the referenceelectrode which varies periodically between an upper value ranging up to1600 mv. and a lower value ranging down to 60 mv., the potentials beingon the hydrogen scale, the upper value of the potential ranging from 900to 1100 mv. for selective etching of rhodium with respect to platinumand gold, 1100 to 1450 mv. for selective etching of rhodium and platinumwith respect to gold and from 1450 to 1600 mv. for etching of rhodium,platinum and gold.

2. Technique for electrochemical etching of a goldtransition metal thinfilm composite having a noble metal selected from the group consistingof platinum and rhodium deposited intermediate the gold and thetransition metal which comprises immersing the composite together withan auxiliary electrode and a reference electrode in an electrolytecomprising an acid having a pH less than one and etching the compositeby applying a difference of potential between the composite and theauxiliary electrode so as to develop a potential on the composite withrespect to the reference electrode which varies periodically between anupper value ranging up to 1600 mv. and a lower value ranging down to -60mv., the potentials being on the hydrogen scale, the upper value of thepotential ranging from 900 to 1100 mv. for selective etching of rhodiumwith respect to platinum and gold, 1100 to 1450 mv. for selectiveetching of rhodium and platinum with respect to gold and from 1450 to1600 mv. for etching of rhodium, platinum and gold.

3. Technique in accordance with claim 2 wherein said potential varies ata periodicity ranging from 1 to 10 milliseconds.

4. Technique in accordance with claim 3 wherein said lower value rangesfrom 550 to --60 mv.

5. Technique in accordance with claim 3 wherein said upper value rangesfrom 1450 to 1600 mv.

6. Technique in accordance with claim 2 wherein said acid ishydrochloric acid.

7. A technique for pattern delineation of a thin film compositecomprising an insulating substrate having deposited thereon a transitionmetal, a noble metal selected from the group consisting of platinum andrhodium and a patterned gold layer which comprises immersing thecomposite together with an auxiliary electrode and a reference electrodein an electrolyte comprising an acid having a pH less than one andetching the composite by applying a difference of potential between thecomposite and the auxiliary electrode so as to develop a potential onthe composite with respect to the reference electrode which variesperiodically between an upper value ranging up to 1600 mv. and a lowervalue ranging down to -60 mv., the potentials being on the hydrogenscale, the upper value of the potential ranging from 900 to 1100 mv. forselective etching of rhodium with respect to platinum and gold, 1100 to1450 mv. for selective etching of rhodium and platinum with respect togold and from 1450 to 1600 mv. for etching of rhodium, platinum andgold.

References Cited UNITED STATES PATENTS 3,639,217 2/19'72 Shukovsky etal. 204l29.43 X 2,890,159 6/1959 Amaya 204l29.43

FREDERICK C. EDMUNDSON, Primary Examiner US. Cl. X.R. l04-129.2

